In integrated circuit design, circuits for commonly used functions, such as AND gates, OR gates, etc., are often combined into "cells", and a model is created for a cell. These models are used in circuit simulation to provide a simpler model of the cell than would be provided by simulating each of the components within a cell. Typically, the model is used to determine the delay of a signal through the cell, the transition time of the output signal and the input and output capacitance values for the cell.
The traditional model for delay of a signal through a cell is a two-coefficient model, similar to the equation for a line, where delay =RD+LD * Ctot. In this model, RD is a number representing the residual, or intrinsic, delay characteristics of the cell, and Ctot represents the capacitive load on the cell multiplied by a constant coefficient LD. Thus, in this model the delay through the cell is assumed to be a function only of the cell characteristics and its capacitive load.
More recently, a three-coefficient equation has been used to represent the delay through a cell. This equation is in the form of delay=RD+LD*Ctot+TD*TRin, wherein RD represents the residual, or intrinsic, delay characteristics of the cell, Ctot represents the capacitive load connected to the output of the cell, multiplied by a coefficient LD, and TRin represents the transition time of the input signal multiplied by a coefficient TD. Thus, in this model the delay through a cell is a function of its intrinsic characteristics, plus the capacitive load on the cell, plus the transition time of the input signal to the cell.
Both these models require an accurate calculation of the capacitive load of the circuits connected to the output of a cell. The three-coefficient model also requires an accurate determination of the input transition time for the signal being input to the cell, which requires an accurate determination of the output transition time of the signal output by another cell that is connected to the input of the cell. Where the transition time of the input signal is very fast for the process, or where the transition time is very slow for the process, the three-coefficient model is less accurate.
There is need in the art then for a method for determining the capacitive load for signals connected to the output of a cell. There is further need in the art for an improved cell model that improves the accuracy for fast and slow transition time signals. The present invention meets these and other needs in the art.